Electrical · Hackbridge, South London

Charging systems tested properly before an alternator goes on

More alternators get replaced than actually fail. A corroded main charging cable, a stretched earth strap, a slipping belt or a battery that was never registered to the car will all produce the same red warning light and the same flat battery on a cold morning. We test the whole charging circuit at our Hackbridge workshop before anyone reaches for a part, and if the alternator is the answer you get a written quote from £390 that includes the belt, since the belt has to come off regardless.

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From £390 · 1.5–4 hours
Mon–Sat 8am–8pm

Mercedes with the bonnet up for electrical work at KITH Cars Garage & Bodyworks, Hackbridge

From £390Written quote before we start

1.5–4 hoursTypical time in the workshop

3 monthsWorkmanship warranty

17 areasCollection across South London

Tested under real electrical load

Everything is switched on before we judge the output, because a failing alternator often looks perfectly healthy at idle with nothing drawing current.

Cables and earths measured too

Voltage drop is checked along the output cable and the earth path. A corroded connection is far cheaper to put right than an alternator, and it lights the same warning.

Labour quoted from your engine

Access runs from under an hour to most of a day depending on where the unit sits. You get the time for your car rather than an average figure before you agree to anything.

The old unit shown to you

The alternator that came off is kept for you to see, and any surcharge on a remanufactured replacement is explained before the bill is written.

In the workshop

How we handle alternator replacement at Hackbridge

Testing happens with the car working, not idling quietly in a bay. The engine runs with headlights, heated rear screen, blower and heated seats switched on, because an alternator with a failed diode pack will hold a respectable voltage at no load and collapse the moment anything is asked of it. At the same time we measure voltage drop along the main output cable and back through the earth path. A few tenths of a volt lost in a corroded terminal is a fault in its own right, and from the driver's seat it looks exactly like a dead alternator.

The oscilloscope shows what a meter cannot. A healthy alternator produces a small, even ripple; one failed diode produces a deep, repeating dip in that pattern, often months before the car ever fails to start. We look at the battery properly too, including whether the vehicle is one of the makes that requires a replacement battery to be registered to the charging system. A battery swapped in without that step gets charged to the wrong strategy, and the symptoms look like a charging fault because in a sense they are one.

Only then does the belt system get the attention it deserves. The tensioner is checked for damping rather than simply for movement, the decoupler pulley on the alternator is checked for the correct free direction, and any oil on the belt is traced to its source before a new one goes on. Fitting the wrong pulley type costs belts and tensioners later, so the replacement is matched to what left the factory. We keep the old unit for you to look at, and where a remanufactured alternator carries a returnable surcharge on the old part, that is explained at quote stage rather than appearing as a line on the invoice.

Electrical

Alternator Replacement: what’s involved

The alternator is driven by the auxiliary belt and generates electricity whenever the engine runs. It powers the car's electrics on the move and recharges the battery after starting. Inside, a regulator controls the output, diodes turn the alternating current it makes into direct current, and brushes feed the rotor. Many diesels and stop-start cars also use a freewheel or decoupler pulley that smooths out engine vibration.

On older cars the alternator simply holds the system at a steady voltage. Many modern cars use smart charging, where the engine ECU varies the alternator's output to save fuel, charging harder when you slow down and easing off when you accelerate. That makes testing more involved, because a reading that would look wrong on an older car may be normal. When an alternator fails, the car runs on battery power alone, usually for a short time before it stops.

Signs your alternator is failing

Battery warning light on

The red battery light while driving usually means the charging system isn't working, not that the battery itself is faulty. Get it checked promptly.

Repeated flat batteries

A car that needs jump-starting, even with a new battery, often has an alternator that isn't charging properly.

Dim or flickering lights

Headlights or dash lights that dim at idle and brighten with revs can point to low or unstable output.

Electrical faults while driving

As voltage drops, systems start to misbehave: warning lights appear, the radio cuts out and power steering or stop-start may stop working.

Whining or grinding noise

A whine that rises with engine speed can be worn bearings or the regulator. A rattle from the pulley can mean a failed decoupler or freewheel pulley.

Belt squeal

A squealing belt may be slipping on the pulley, which stops the alternator charging properly. It can also point to a worn tensioner.

Hot or burning smell

An overheating alternator or a slipping belt can give off a hot electrical or rubber smell and should be checked straight away.

When should it be done?

Alternators don't have a set replacement interval. Many last well over 100,000 miles, but bearings, brushes, regulators and pulleys wear, and oil or coolant leaks can shorten their life. Have the charging system tested if the battery light comes on, the battery keeps going flat or you hear a whine from the front of the engine. It is also worth checking whenever a new battery is fitted, so a charging fault isn't mistaken for a bad battery.

Battery warning light on while driving
Flat battery that returns after recharging or replacement
Whining, grinding or rattling from the alternator or its pulley
Oil or coolant leaks onto the alternator
Whenever a new battery is fitted

How we replace an alternator

Check-in and symptomsWe ask about warning lights, flat batteries and noises, and whether a new battery has been fitted recently.
Charging system testWe test battery health, alternator output under load and voltage ripple, and read any charging codes from the ECU, allowing for smart charging systems.
Check belt, pulley and wiringWe inspect the auxiliary belt, tensioner and pulley, and check the main charging cable and earth connections. A bad earth can look like a failed alternator.
Written quoteWe show you the results and give you a written quote. Work only goes ahead once you approve it.
Remove the old alternatorWe disconnect the battery, remove the belt and take out the alternator. On some engines this means removing other components for access.
Fit the new unit and beltWe fit the new alternator and a new auxiliary drive belt, check the tensioner and idler pulleys, and tighten the terminals to the correct torque.
Retest and road testWe retest the charging voltage at idle and under electrical load, clear any codes, and road test the car to confirm correct charging.

Parts we fit

We fit new or quality remanufactured alternators to OE specification, from reputable manufacturers such as Bosch, Valeo or Denso. The output rating and pulley type must match the original, especially on stop-start and smart charging systems. We always fit a new auxiliary drive belt, from suppliers such as Gates, Dayco or Continental, and replace the tensioner if it is worn.

New or remanufactured OE-specification alternators
Correct freewheel or decoupler pulley where fitted
New auxiliary drive belt
Belt tensioner and idler pulleys where worn
Charging cable and earth repairs as required

How long it takes

1.5–4 hours

Many alternators can be replaced in a couple of hours. On some engines the alternator sits low down or behind other components and takes considerably longer. We will give you a time with your quote.

What’s included

Full charging system test before replacement
Battery health check
New auxiliary drive belt fitted
Tensioner and pulley inspection
Charging system retest after fitting
Road test

Worth knowing

What people get wrong about alternator replacement

Fitting a battery without registering it

Several makes need a new battery coding to the car. Skip that and the charging strategy stays set for the old one, which shortens the new battery's life and muddies any diagnosis that follows.

Blaming every whine on the alternator

The power steering pump, air conditioning compressor, tensioner and idler pulleys all live on the same belt. A few minutes with a stethoscope identifies which one is singing before anything is unbolted.

Jump-starting week after week

Each deep discharge takes something permanent out of a battery, and a car that starts on a jump tells you nothing about why it went flat. Having the charging system tested once works out cheaper.

Expecting a long drive to recharge it

On a smart-charging car the ECU deliberately limits output once it decides the battery is in reasonable condition. A deeply flat battery usually needs a proper bench charge instead.

Instant quote

Price your alternator replacement now

Enter your reg and we’ll look up your car, then price the job from our own labour rates and the book time for your model. Nothing is booked until you say so, and the written quote comes before any work.

Free, about 30 seconds, no sign-up
Parts, labour and VAT included
3-month workmanship warranty
Collection and delivery across South London

“It’s rare to find a garage you can truly trust. Fair price, brilliant customer service.”Thinushan K. · ★★★★★ Google

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Questions

Alternator Replacement: your questions answered

What if the car has already stopped?

We run recovery ourselves, so a car that has died at the roadside or will not start on the drive can be collected and brought in. It is often the sensible option, because pressing on with a car that is not charging risks losing lights and power steering somewhere far worse.

Can I supply my own alternator?

We will fit a part you have bought, but it is worth knowing where that leaves you. Our 3-month workmanship warranty covers the fitting; if the unit is faulty out of the box, the labour to remove it and fit another is chargeable again. On many engines that labour is the larger part of the bill.

Will I need a new battery as well?

Not always. A battery dragged down by a failing alternator sometimes recovers and sometimes does not, so we test it once the new unit is in and charging rather than adding one to the quote automatically. If it fails the test, we will show you the result.

How long will the car be with you?

Testing takes around half an hour and most replacements are same-day where the part is available. On engines with the alternator buried under the inlet manifold or behind other components it can take most of a day, and we will tell you which yours is when we quote.

Could it just be the battery?

It could. Batteries and alternators produce similar symptoms, which is why we test both, along with the belt and wiring. A new battery on a car with a failing alternator will soon go flat again.

Why replace the drive belt too?

The belt has to come off to change the alternator, and an old belt is often glazed or cracked. Fitting a new one at the same time costs little extra and avoids paying for the same labour again when it fails.

Can I drive with the battery light on?

Only a short distance, and with as few electrical loads on as possible. The car is running on battery power and could stop without much warning. If the light came on with a squeal or loss of power steering, the belt may have failed, so stop and get advice.

New or remanufactured?

A quality remanufactured alternator is rebuilt with new wear parts and tested to OE specification, and is a sensible choice for many cars. We'll tell you which options are available for yours and the price of each.

What is smart charging?

On many modern cars the engine ECU controls alternator output, reducing it when the battery is well charged and increasing it when you slow down, to save fuel. It means voltage readings vary more than on older cars, so testing needs to take this into account.

Alternator Replacement near you

We look after alternator replacement for drivers right across South London. Pick your area for local prices, directions and collection:

Beddington~6 min away Carshalton~5 min away Cheam~12 min away Coulsdon~17 min away Croydon~15 min away Mitcham~8 min away Morden~12 min away Norbury~18 min away Purley~15 min away Rosehill~9 min away Streatham~20 min away Sutton~10 min away Thornton Heath~15 min away Tooting~18 min away Wallington~6 min away Wimbledon~20 min away Worcester Park~20 min away

Related work we do

Battery & Electrical RepairsFrom £180 · 30 minutes–2 hoursBattery testing and replacement, starting and charging checks, and electrical fault finding for all makes.Engine DiagnosticsFrom £122 · 1–2 hoursA proper diagnosis of warning lights, running faults and electrical problems, not just a code read.Cambelt ReplacementFrom £475 · 3–6 hoursCambelt (timing belt) replacement with tensioner, idlers and, where belt-driven, the water pump.Car ServicingFrom £155 · 1–4 hoursInterim, full and major servicing for all makes, carried out to the manufacturer's schedule with the correct specification oil.MOT TestingFrom £50 · 45–60 minutesClass 4 MOT testing for cars and light vans at a flat £50, under the DVSA maximum fee of £54.85, with nothing added for booking or admin.Brake RepairsFrom £110 · 1–3 hoursBrake pad and disc replacement, calliper and slide pin servicing, brake fluid changes and hose and pipe repairs for all makes.